High mobility photoconductor sintered shapes and process for their preparation



4, 1965 A. REISMAN ETAL 3,202,609

HIGH MOBILITY PHOTOCONDUCTOR SINTERED SHAPES AND PROCESS FOR THEIR PREPARATION Filed Jan. 31, 1962 FIGW WEN UUUUHU/ 3 UU U UH U U H H H U H H H H H 7 J DIFFU- FIG 2 PUMP MECHAN- ICAL PUMP 10 02 N2 FIG. 3 l l K J INVENTORS 15 15 4 MELVIN BERKENBLIT H a H fi H R g H GEORGE CHEROFF if? :Tti: FREDERICK HOCHBERG ARNOLD REISMAN A I BY e u u ju u u 5 ug u 2 12 ATTORNEY Hum MOBILITY rHorocoNDUCToR SINTERED snArEs AND, PROCESS FOR THEIR PREPARA- rroN Arnold Reisman and Melvin Berkenblit, Yorktown Heights, and George Cherolf and Frederick Hocliberg,

Peelrslriil, N.Y., assignors to International Business Machines Corporation, N ew York, N.Y., a corporation of New York Filed Jan. 31, 1962, Ser. No. 170,015 2 Claims. (Cl. 252-501) V.

This process relates to the preparation of photocon ductor sintered shapes. More particularly, it relates to the preparation of high effective ,mobility cadmium selenide sintered shapes Without the use of a flux material or substrates. These cadmium selenide sintered shapes exhibit effective mobilities many times greater than observed heretofore in fluxed cadmium selenide sintered layers.

Cadmium selenide sintered shapes that are self supporting and have not been sintered via a fluxing operation were previously unavailable. Electrical devices prepared from cadmium selenide have previously been made by sintering the CdSe in the presence of a flux on a substrate. Such sintered CdSe photoconductors will generally exhibit effective mobilities in the range 80 -100 cmF/volt sec. A photoconductor ideally should exhibit a maximum sensi- 3,202,609 Patented Aug. 24, 1965 ice ment of the invention, as illustrateed in the accompanying drawing.

FIGURE 1 is a View, partly in cross section of the sealed evacuated apparatus used in the equilibrium sintering step of the process showing the compressed CdSe bodies on the alumina slab.

FIGURE 2 is a view, partly in cross section, of the open constantly evacuated apparatus system used in the steady state sintering step of the process showingthe compressed CdSe bodies on the alumina slab.

FIGURE 3 is a view partly in cross section, of the apparatus used in the ambient atmosphere equilibrium firing of the process showing the compressed CdSe bodies on the alumina slab.

,Cadmium selenide photoconductor sintered shapes exhibit an effective mobility which is many times greater than the effective mobility of fiuxed cadmium selenide sintered layers and are highly desired for use in photo conductor devices. The efliective' mobility of a photo conductor device is a parameter which is related to two measurable quantities, sensitivity and speed. This param eter is a figure of merit used in judging the effectiveness of the photoconductor device when it is to be used in logical functions in circuit applications. The relationship of speed and sensitivity to effective mobility of a photoconductor device is expressed in Equation I.

tivity (ratio of dark/light resistancel and a minimum' etries of a fabricated device and has limited the tem-' perature range over which a photoconductor can be fabricated because of impurity contaminationvia reaction of the flux and the. substrate with" ultimate incorporation of'these impurities in the sintered layer. e

The process of the invention prepares cadirnum selenide sintered shapes which exhibit effective mobilities many times greater than those heretofore observed in fluxed sintered, layers. In addition the sintered shapes are self supporting enabling the aflixing of electrical contacts to any of the surfaces of the sintered shapes rather than -t o the single exposed surface of a fluxed sintered layer.

It is an object of the invention to provide a process for preparing photoconductors of high effective mobility.

, It is another object of the invention to prepare cadmium selenide sintered shapes which are self supporting. It is a further object of the invention to prepare sintered cadmium selenide shapes of high effective mobility and such that the sintered shapes are self supporting and have all their surfaces available for aflixing'electrical contacts. Another object of the invention is to prepare cadmium selenide photoconductor material with high effective mobilities many times greater than those observed in fluxed sintered layers and which are self supporting.

The foregoing and other, objects, features, and advantages of the invention will be apparent from the following more particular description of a proven embodi- (Equation I) volts, I is the light intensity in watts per square centimeter, 1

equals the initial slope of the decayflcurve" conductance (G) versus time (t), w is electrode gap, 1 is electrode length, and a is effective mobility expressed in cmfl/volt sec. The term i i o Y Y dG- specifically contains the quantities decay time and sensitivity (which is measured as' conductance). The remaining quantities are all constants for-a given set of conditions. Thus, a smaller decay time, coupled with a more sensitive material results in a greater effective mobility.

r The polycrystalline cadmium selenide powder used in the process of the invention must be analytically stoichiometric, stoichiometric within the limits set by wet chemical and X-ray analysis, but preferably should exhibit the proper deviation from stoichiometry commensurate with the sintering temperature used in the fabrication of the photoconductor element. I

The process of the invention can best be described as follows:

A- suitable impurity, such as for example copper-- added as copper chloride (CuCI is mixed with cadmium selenide polycrystalline powder. The powder is then pressed into any desired shape (for example a disc or'pellet) at pressures from 25,000 to 100,000 lbs. per square inch. t a

The pressed shapes of cadmium selenide 1 are placed on a slab (which may be alumina or quartz or other suitable inert material depending upon the temperature to which it will be subjected) and the slab placed in a quartz 4 EXAMPLE 1 2 g. of analytically stoichiometric CdSe 400 mesh polycrystalline powder is admixed with .0063 g. of CuCl dissolved in triple distilled water after which the powder is i 3 which is impervious to Cadmium .selenide and dried at 110 for 2 hours. The 2 g. of analytically stoimlum and sglemum vapor: The tube 15 evacuated to a chiometric 400 mesh CdSe polycrystalline powder is adpressure from 1 to 7X1.0 of mercury and h mixed with suificient Cu present as CuCl to give a matesealed. The sealed tube is then heated (e.g. by a heating rial containing 175 mm of Cu by Weight The dried c011 4) at from 700 to 1100 for Several hours powder is completely mixed in a ball mill and pressed into hours), cooled to room temerawr? and broken PF f a circular disc shape at 75,000 lbs/square inch. The disc remove the slab and cadmium selenide shapes. This iniis placed on an alumina slab and the l is sealed into tial sintering step in a closed evacuated. tube is hereby an evacuated quartz tube at 7 1()6 mm f This defined as the equilibrium sintering step of the process. tube i heated at 900 C, for 2 hours, After the equilib- Subsequent to this step the slab 2 containing the CdSe rium sintering is completed the tube is cooled to room shapes 5 are now placed in a quartz furnace tube6 which temperature and broken open. Next the discs and slab is impervious to cadmium selenide and cadmium and are Subjected to steady state sintering at 600 C. for 2 selenium vapors. It contains a thermocouple Well 7 for hours In a contlnuous Va1 111m at P0355111"e 0f X monitoring the temperature. The furnace tube of quartz of Hg and cooled again to T0011} f lf is sealed at one end and open to a vacuum system at the sequenfly the OT'NZ atmosphere equlhbmtlon takes P13Ce other in a 1% 0 -99% N atmosphere at 535 C. for 2 hours. A heating coil element 4 encompasses the furnace tube After q i i the shapes are glectroded and ghlblt effec' tive mobilities of 20,000 cm. /volt see. A sintered layer in the region in which the slab with the CdSe shapes to be h t d 1 t d Th t 8 h ad t prepared by fluxing techniques from the same starting ea 6 are Oca e ere ls a 10m w 1c Is 0 material yielded an effective mobility of 105 cmP/volt sec. connect the furnace tube to a cold trap 9 cooled by liquid N 10 which protects the vacuum systems by collect- EXAMPLES 245 s y 10W vapor fractions- The vacuum system The procedure of-Example 1 is repeated except the prises a diffusion pump and a mechanical (fore) pump. process parameters are varied as indicated below in The fore pump reduces the pressure to a low range and Table I.

Table I Equilibrium Steady state O -N Atmosphere sintering (closed sintering (open Equilibration Efiectlve Compactvacuum system) Pressure vacuum system) Pressure Efiective mobility of Ex. N o. iug (mm. of (mm. of mobility of fluxed sintered pressure Hg) Hg) sintered shapes layers (cmfi/ (lb/in?) Temp. Time Temp. Time 1% 0 -99% Gas mixture (cmJ/volt sec.) volt sec.)

( 0.) (hr.) 0.) (hr.) N z Tempera system Time ture 0.) (hr.)

25, 000 000 2 1 i0- 535 1 3 10 535 1 3, 000 so 90, 000 800 2 4x10 600 3 1 10 535 3 700 00 60,000 1, 100 1 3x10 575 2 2 i0 535 2 50, 000 110 75, 000 700 s 7x10 600 5 7 10 535 4 16,000 95 100,000 1, 000 3 IXIO- 500 3 (10- 535 5 12, 000 q 115 the diifusion pump takes it down to the ultimate vacuum range.

The furnace tube 6 is continuously evacuated to a pressure of 17 10- mm. of mercury for 1-5 hours at 500- 600 C. The slab and CdSe shapes are then cooled to room temperature. This process step of heating in an open evacuated system is hereby designated as steady state sintering.

The next process step is O -N atmosphere equilibration heat treatment. This step involves heating the sintered CdSe shapes on the slab in a 1% 0 -99% N atmosphere at 1 atmosphere pressure for 1-5 hours at 535 C. The slab 2 with the sintered CdSe shapes 1 are placed in a quartz tube 12 which is sealed at both ends with plugs 13 made of tetrafluoroethylene (Teflon) through which a tube 14 is placed for effluent flow of the O -N gas mixture. The O and N are introduced into the tube by means of a Y shaped tube 15 located above the slab containing the CdSe sintered shapes. A heating coil 4 encompasses the tube.

The quartz tubes used in any of the process steps may be of any diameter or any shape, but preferably tubes are used which are commensurate with the quantity of material which is to be treated.

The sintered shapes are then cooled down and electroded. The eiiective mobility can vary from 700 to 50,000 cm. /volt sec. These electroded shapes can be utilized as the photoconductor element in light sensing circuits or as optical relays. When said relays are combined in various arrays they can be used to perform logical functions in computer circuitry.

The CdSe sintered shapes prepared according to the process of the invention may be used as active components in performing logical functions in computer circuits.

The process of the invention discloses the preparation of high mobility polycrystalline cadmium selenide sintered shapes. It involves the introduction into OdSe of copper as a dopant for example by means of CuCl;. The powdered mixture is pressed into shapes,.sealed in quartz and subjected to equilibrium sintering. The shapes are then subjected to steady state sintering at a lower temperature in a continuous vacuum. Next O -N atmosphere equilibration heat treatment is conducted at a still lower temperature. The shapes are cooled and then electroded. Effective device mobilities for these sintered shapes are observed to be many times greater than those observed in fluxed sintered layers prepared from the same copper doped level of cadmium selenide.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. Cadmium selenide photoconductor sintered disc shapes having a high effective mobility of about 700- 50,000 cm. /volt sec. prepared by process which comprises:

(1) mixing cadmium selenide polycrystalline powder with suificient copper as in solution to yield a material containing to 400 parts per million of the copper by weight;

(2) pressing the doped cadmium selenide powder into disc shapes at a pressure from 25,000 to 100,000 pounds per square inch;

I (3) equilibrium sintering the pressed discs at a temperature between 700-li100 C. for from 1 hour to 8 hours in a vacuum at a pressure from 17 10- mm. of mercury;

(4) then steady state sintering these discs at a temperature of 500-600 C. for 1 to 5 hours in a vac uum at a pressure from 1-7 10- mm. of mercury;

(5) now subjecting the sintered discs to oxygen-nitrogen atmosphere equilibration heat treatment at 535 C. for 1 to 5 hours in an atmosphere (if 1% oxygen and 99% nitrogen at one atmosphere of pressure;

(6) and then subsequently air quenching the sintered shapes to room temperature.

2. A process of'preparing cadmium selenide photoconductor sintered shapes having high effective mobility of about 70050,000 cm. volt sec. which are self supporting and have all surfaces available for aflixing electrical contacts which process comprises:

(1) mixing cadmium selenide polycrystalline powder with suflicient copper present as copper chloride in solution to yield a material containing 100 to 400 parts per million of copper by Weight;

(2) pressing the mixture thus obtained into desired shapes at a pressure from 25,000 to 100,000 pounds per square inch;

(3) equilibrium sintering the pressed shapes at a temperature between 700-1100 C. for from 1 hour to 8 hours in a vacuum;

(4) then steady state sintering'these shapes at a temperature oft500-600 C. for 1 to 5 hours in a vacuum;

(5) now subjecting the sintered shapes to oxygen-nitrogen atmosphere equilibration heat treatment at 535 C. for 1 to 5 hours in an'atmosphere of 1% oxygen and 99% nitrogen; v

(6) and then subsequently cooling the sintered shapes.

References Cited by the Examiner UNITED STATES PATENTS 2,765,385 10/56 Thomsen 252501 2,843,914 7/58 Koury 252501 2,908,594 10/59 Briggs 252-501 2,938,003 5/60 Jacobs 252 501 JULIUS GREENWALD, Primary Examiner.

JOSEPH R. LIBERMAN, Examiner. 

1. CADMIUM SELENIDE PHOTOCONDUCTOR SINTERED DISC SHAPES HAVING A HIGH EFFECTIVE MOBILITY OF ABOUT 70050,000 CM.2/VOLT SEC. PREPARED BY PROCESS WHICH COMPRISES: (1) MIXING CADMIUM SELENIDE POLYCRYSTALLINE POWDER WITH SUFFICIENT COPPER AS IN SOLUTION TO YIELD A MATERIAL CONTAINING 100 TO 400 PARTS PER MILLION OF THE COPPER BY WEIGHT; (2) PRESSING THE DOPED CADMIUM SELENIDE POWDER INTO DISC SHAPES AT A PRESURE FROM 25,000 TO 100,000 POUNDS PER SQUARE INCH; (3) EQUILIBRIUM SINTERING THE PRESSED DISCS AT A TEMPERATURE BETWEEN 700-1100*C. FOR FROM 1 HOUR TO 8 HOURS IN A VACUUM AT A PRESSURE FROM 1-7X10**-6 MM. OF MERCURY; (4) THEN STEADY STATE SINTERING THESE DISCS AT A TEMPERATUR OF 500*-600*C. FOR 1 TO 5 HOURS IN A VACUUM AT A PRESSURE FROM 1-7X10**-6 MM. OF MERCURY; 